The present invention relates to a method for the purification of bidentate organophosphorous compounds. More specifically the invention relates to a method for purifying bidentate organophosphorous compounds such as dihexyl N,N-diethylcarbamylmethylenephosphonate by mercury precipitation.
Solvent extraction processes which employ bidentate organophosphorous extractants are presently being actively developed by U.S. Department of Energy facilities at Hanford, Washington and Idaho Falls, Idaho for the removal and recovery of the antinide elements, particularly americium and plutonium, from nitric acid waste solutions generated at these sites. In general bidentate organophosphorous compounds are efficient extractants for trivalent, tetravalent, and hexavalent actinides which are present in aqueous acidic nuclear waste solutions. With the bidentate extractant, essentially all actinide values, e.g. AM (III), Cm (III), Pu (IV), Np (IV) and U (VI) are extracted into the organic phase and, thereafter, the actinides are stripped from the organic by contact with dilute aqueous acidic or alkaline solutions. U.S. Pat. No. 3,993,729, issued Nov. 23, 1976 and assigned to the common assignee describes the application of bidentate organophosphorous extractants and actinide removal schemes from various acid nuclear fuel reprocessing solutions such as those generated in the Purex process.
Currently, preferred bidentate extractants are dihexyl-N,N-diethylcarbamylmethylenephosphonate hereinafter referred to as DHDECMP, and dibutyl-N,N-diethylcarbamylmethylenephosphonate, hereinafter referred to as DBDECMP. These extractants and other neutral bidentate organophosphorous compounds are generally available as crude products (about 65% pure) which contain various impurities. The actinides are irreversibly extracted from acidic waste solutions using crude DHDECMP as the extractant. The actinides are readily stripped from purified DHDECMP using dilute nitric acid which contains hydroxylamine nitrate to reduce plutonium to valence (111). Thus, DHDECMP or DBDECMP must be purified to permit the use of low acid solutions to strip actinides from these extractants.
Present methods for purifying the commercially available DHDECMP and DBDECMP from the unknown impurities include vacuum distillation, liquid chromatography and hydrolysis of impurities at 60.degree. C. with a 6 M HCl solution. Another method for purifying the bidentate organophosphorous compounds is described in U.S. Pat. No. 4,051,203, which issued Sept. 27, 1977 and is assigned to the common assignee. As described herein, the bidentates in an organic solvent are purified by a contact with ethylene glycol which perferentially extracts the impurities away from the solvvent solution.
However, all of these purification procedures have disadvantages. Specialized vacuum distillation and liquid chromatography equipment is expensive and DHDECMP or DBDECMP obtained from these methods cost more than twice as much as the technical grade material. Specialized corrosion resistant equipment is needed to perform HCl hydrolysis of impurities present in the crude bidentate extractants, and more importantly the HCl also attacks the bidentates as well as the impurities. The use of ethylene glycol for purification does not seem to produce a product that has all of the desirable extraction characteristics. Therefore, there remains a need for a simple, reliable and effective purification process which can be carried out economically and easily with conventional equipment.